In general, inkjet printers include at least one printhead that ejects drops of liquid ink onto a surface of an image receiving member. A phase change inkjet printer employs phase change inks that are solid at ambient temperature, but transition to a liquid phase at an elevated temperature. The melted ink can then be ejected onto the surface of an image receiving member by a printhead. The image receiving member may be a media substrate or an intermediate imaging member, such as a rotating drum or endless belt. The image on the intermediate imaging member is later transferred to an image receiving substrate. Once the ejected ink is on the surface of the image receiving member, the ink droplets quickly solidify to form an image.
Phase change inkjet printers typically employ melting devices having one or more heated plates that melt solid phase change ink contacting the plate and deliver the melted ink to an associated printhead. The melting devices use high watt densities to rapidly heat the melt plates with associated heater elements and to provide a flow of ink to the printheads at a specified rate and temperature. This rapid heating of the melt plates, however, can cause delamination or damage to the heater elements or the melting device circuit. The problems associated with rapid heating are compounded when an uneven thermal load exists over the heated surfaces of the melt plates. For example, an uneven thermal load can occur when some regions of the melt plates are in direct contact with the solid ink and other regions are in contact with only a residual film of previously melted ink or no ink at all. Films of ink remaining outboard of the regions of the melt plates in direct contact with the solid ink can be damaged from the rapid heating.
Existing solutions to the problems associated with rapidly heating melt plates subject to non-uniform thermal loads suffer from a number of drawbacks. For instance, one solution entails providing two separate heaters and two separate heater circuits to separately control the heating of the different regions of the melt plates. This solution, however, adds significant cost to the production of the printer. Another solution is to reduce the overall power to the region of the heater that is not in contact with the thermal load. This solution becomes problematic as the melt temperature of the ink and the required drip temperature off the melt plate grow farther apart. The task of raising the molten ink to the desired drip temperature falls to the region of the melt plate having a lesser thermal load, requiring an elevated watt density to keep up with increasing ink flow rates.
What is needed, therefore, is a heater device that utilizes a cost effective single channel circuit to drive at least two heated regions with different thermal loads in an inherently safe and heat-load-balanced system. A heating device that can be operated with an effective voltage control that enables rapid initial heating of the melt plates with a high voltage followed by sustained operational heating of those plates with a reduced voltage after warm-up to prevent heater or ink damage is also desirable.